This invention concerns measurment of depths in subsea wells and, particularly, measurement of the distance between a submerged wellhead and a downhole tubing hanger. When completing subsea wells, downhole tubing hangers are installed 300 to 700 feet below the wellhead. The tubing between the downhole hanger and the wellhead is run as part of the christmas tree and has a tie-back tool that connects to the downhole hanger. When the christmas tree is latched to the wellhead, the tie-back tool must also make up and latch to the downhole tubing hanger. If the tubing is too short, it will never reach down to the hanger. On the other hand, if the tubing is too long, excessive compressive stresses might be placed in the tubing when the christmas tree is locked down onto the wellhead since the extra length of tubing must be accommodated in the short, 300 to 700 feet, interval. Consequently, it is important to know the exact length of tubing required between the downhole hanger and the wellhead.
One way of determining the length of tubing required has been to run a plate member on drill pipe after the blowout preventers have been removed. The plate is positioned on the drill pipe so that it will land on the wellhead when a protective pad on the drill pipe tags the downhole hanger. The plate is then locked to the drill pipe at that height. The drill pipe is retrieved and the distance between the plate and the protective pad is measured and used to space out the proper tubing length.
That operation is time consuming and exposes the well to undesirable risks, including (1) the potential for damage to the wellhead when stabbing the drill pipe into the open wellhead, (2) the well is open to the sea and totally dependent upon the integrity of the cemented production strings for a considerable length of time, and (3) potential for damage to the downhole hanger by driller error or excessive vessel heave during the tagging of the hanger. Therefore, a means for very accurately measuring the distance between the downhole hanger and the wellhead which avoids the foregoing risks was needed.
One suggested solution to the problem is to run a tool on the lower end of an electric line with the blowout preventers still in place. The tool transmits a signal to the surface when it is lowered to and tags the downhole hanger. Directly reading the amount of cable paid out is not accurate enough because existing surface measurement equipment cannot achieve the accuracy needed. Cable slippage on the measurement sheave is one problem. Even though wave compensation equipment is used on the drill ship, some motion is transmitted downhole to the tool since it is recording over a long distance and time interval. The best accuracy achievable by this method appears to be about one foot.
The method of the present invention achieves the desired measurement accuracy, .+-.1/2 inch in 700 feet, while avoiding all of the aforementioned risks.